To improve high temperature stability and to provide better physical and electrical properties over amine cured epoxy resin systems, carboxylic acid anhydride curing agents have been found to be particularly useful with epoxy resins for high voltage insulation applications. When carboxylic acid anhydrides are used, an accelerator is usually required to give reasonable gel times at elevated temperatures. For example, Smith, et al., in U.S. Pat. No. 4,026,862, taught an epoxyanhydride insulating composition cured by separate addition of minor amounts of a quaternary onium salt, such as benzyltrimethylammonium iodide, as a latent catalyst. This composition could optionally contain an imidazole, such as 1-methylimidazole, acting as an additional latent catalyst. The system also required the use of a carboxylic acid stabilizer, to improve storage stability.
Because of higher performance requirements now demanded of epoxy resins in insulation systems and reinforced plastics, there is an ever increasing need for improved catalysts, curing agents, and insulating compositions. What is needed is an insulating composition having improved heat distortion temperatures, tensile strength, and thermal stability, particularly for high temperature bracing and insulation.
Bacskai, in U.S. Pat. No. 4,011,282, in an attempt to provide higher performing epoxies, used a copolymer of maleic anhydride and dihydrofuran, as a new type of curing agent for epoxy resins. However, the use of components having only carbon atom ring structures containing oxygen, did not provide the extreme thermal stability required for advanced technology high temperature bracing and insulation.